1. Field of the Invention
The present invention relates to nucleic acid markers for rice blast resistance genes, and rice blast resistance genes isolated by using these nucleic acid markers.
The rice blast resistance genes are very useful not only for the development of superior cultivars of rice, but also as a material for research and a genetic resource for creating new resistance genes capable of being introduced into various plants.
2. Description of the Related Art
It is well known that there are genes rendering resistance against pathogens to plants, and introduction of these genes has been an important target in the conventional breeding efforts. As a result, many new types of cultivars have been created to date through introduction of these resistance genes. Importance of these resistance genes will further increase throughout the world since such biotic methods utilizing the innate functions of plants to prevent epidemics will lower consumption of chemical pesticides, promote human health, protect the environment while still lowering the cost of agriculture.
Among the plant resistance genes against pathogens, the rice blast resistance gene was first discovered in Japan, and the presence of many such genes has since been discovered. In particular, resistance genes derived from indica rice exhibit resistance to many strains of the rice blast fungi found in Japan, and are highly useful as genetic resources. Among others, Pi-ta and Pi-ta.sup.2 genes derived from indica rice are suited for RFLP mapping of the genes. There are however still only a very limited number of cases of actual introduction of these resistance genes into present elite cultivars.
The probable reasons are that the conventional breeding method requires many generations of backcrosses to introduce a resistance gene into a cultivar, accompanied with resistance tests by inoculating pathogens to many individuals every year. Application of a resistance test to pathogens of foreign-origin is almost impossible within Japan because of the strict control over the importation of foreign pathogens.
On the other hand, recent progress in plant biotechnology has enabled the identification and isolation of various genes, and the introduction of them into other plants. Therefore, for plant resistance genes also, it is not difficult to introduce them into any desired cultivar by genetic engineering techniques if it was cloned. Further, it will drastically reduce the time and labor required for breeding resistant cultivars. It will also be possible to clarify the mechanism about how the resistance genes work in plants, and make possible modifications to the present gene and then provide new types of resistance genes. Many research groups are now making efforts to isolate resistance genes, but only a few have been successful to date. This is attributable to the fact that there is only limited information about the biochemical character of the resistance gene-products.
As a method for identifying and isolating genes, a technique known as positional cloning is now attracting attention. This technique uses nucleic acid markers near a target gene in a genome map, and isolates target genes from a genome library. Actually, some genes causing human hereditary diseases have been isolated by the application of this technique.
Although only a few cases of success of this positional cloning have been reported in plants, rice is considered to be the most suited plant for this technique for the following reasons: (1) rice has the smallest genome size among the major crops; (2) physical distance (in kb) corresponding to an unit genetic distance (cM) is very small in rice; (3) it is easy to limit the range of the gene location by utilizing several near isogenic lines (NIL) which have been developed by introgressing the indica derived genes into japonica backcross; and (4) it is easy to introduce genes into cells for a complementation test.
An important key to success of this positional cloning is whether or not good adjacent markers are available. Calculating from the rice genome size and the genetic map, the physical distance corresponding to 1 centiMorgan (cM) of the rice genetic map is estimated to be about 100-200 kb on a nucleic acid basis. On the other hand, the average size of the insert of yeast artificial chromosome (YAC) is more than 200 kb. Therefore, if there is a DNA marker for the resistance genes within a distance of 100 kb, i.e., 0.5-1 cM, the possibility of success of positional cloning of the gene is considered to be very high.
Such nearby nucleic acid markers of rice blast resistance genes would be also useful for largely reducing the time and labor required for testing resistance in conventional breeding through backcrosses, for example.